1. Field of the Invention
This invention relates to a color image pickup system and, more particularly, to a system which utilizes at least a part of the image pickup apparatus for automatic adjustment of white balance.
2. Description of the Prior Art
Heretofore there have been proposed various color image pickup systems of the type having an automatic white balance adjusting function. FIG. 1 of the accompanying drawings shows an example of the conventional color image pickup system having the automatic white balance adjusting function. In this example, primary color signals for red, green and blue (hereinafter called signals R, G and B respectively) are obtained from an image pickup element. Referring to FIG. 1, the signals R, G and B, which are image pickup outputs of the image pickup outputs of the image pickup element 1, are supplied to processing amplifiers 5, 5' and 5" respectively through sampling hold circuits 2, 2' and 2" (hereinafter called S/H circuits) and pre-amplifiers 3, 3' and 3". The outputs of the processing amplifiers 5, 5' and 5" are supplied to a matrix circuit 6. The matrix circuit 6 forms a brightness signal (hereinafter called the signal Y) and color difference signals R-Y and B-Y respectively from the primary color signals. The color difference signals R-Y and B-Y are supplied to DC restorers 8 and 8' which respectively include low-pass filters and clamping circuits. The outputs of the DC restorers 8 and 8' are supplied to a color encoder circuit 9 together with the above-stated signal Y.
At the color encoder circuit, the signal Y has a synchronizing signal added thereto and is clipped. Meanwhile, the color difference signals are subjected to a modulation process, etc. Then, the signal Y and the color difference signals are mixed into a color television signal, which is produced from the terminal 11 of the encoder circuit 9. A synchronizing signal circuit arrangement 10 produces a blanking signal, a chrominance subcarrier, etc. as well as the synchronizing signal.
There are provided gain control circuits 4 and 4' which are disposed on the transmission routes for the signals R and B and which adjust white balance by controlling the levels of the signals R and B with the signal G used as a reference. The outputs of the DC restorers 8 and 8' are supplied to comparators 14 and 14' through smoothing circuits 12 and 12'. The comparators 14 and 14' cause up-down counters 15 and 15' (hereinafter called U/D counters) to count upward when the levels of these signals are lower than the reference voltage level produced from a reference voltage generator 13 and to count down when their levels are higher than the reference voltage. The outputs of the U/D counters 15 and 15' are supplied to the gain control circuits 4 and 4' through memories 16 and 16' and digital-to-analog converters 17 and 17' (hereinafter called the D/A converters). This determines the gain of each of the circuits 4 and 4'.
When the operator of the system issues a control instruction for white balance by turning on a switch which is not shown, the image pickup element 1 produces a signal corresponding to a white or gray color. If the operator then directs the image pickup element 1 toward a white object or places a white transmitting diffusion plate or the like in a picture-taking light flux, the gain control circuits 4 and 4' control and adjust the color difference signals R-Y and B-Y to a zero level through the processes described above. In other words, the white balance is adjusted with the output signals of the U/D counters 15 and 15' used as control signals. After that, with the control signals supplied to the memories 16 and 16', the gains of the gain control circuits 4 and 4' are determined by white balance control signals stored in the memories 16 and 16' during an image pickup operation.
The color image pickup system described above has been used as a camera for a video tape recorder (hereinafter called VTR). In the meantime, there has recently been proposed a system (hereinafter called the SV system) which gives color still pictures by discretely extracting one-field portions of a color picture signal and by recording them in circular tracks on a magnetic recording sheet, so that color still pictures can be obtained by continuously reproducing these circular tracks.
Generally, the SV system is not operated continuously over a relatively long period of time but is operated over a relatively short period of time. For example, the SV system is operated to obtain several pictures at a time in the same manner as in ordinary photographic cameras using silver salt films.
If the conventional arrangement is adopted for the SV system, various problems with regard to white balance adjustment arise as described below.
In order to satisfactorily accomplish white balance adjustment in the SV system, the white balance must be adjusted immediately before every image pickup operation. However, since the SV system is intended to be used in the same manner as in conventional photographic cameras, such white balance adjustment that is to be carried out immediately before every image pickup operation tends to result in a failure to seize opportunities for picture coloring. A conceivable solution to the problem is to have power constantly supplied and to carry out an image pickup operation by using control data stored in memory. Such an arrangement, however, not only results in increased power consumption but also tends to carry out an image pickup operation on the basis of control data which has already become invalid and inadequate. Besides, in such a situation, an important scene might inevitably be recorded with a faulty signal while the operator is not aware of inadequate control data.
An object of the present invention is to provide a color image pickup system having a white balance adjusting function which is highly suited for the above SV system.
Another object of the invention is to provide a color image pickup system which is capable of giving a picture signal of acceptable white balance even in situations where an image pickup operation is carried out for the first time or after the lapse of a very long period during which the system has remained unused. The system uses, in such a case, prescribed initial data as control data for controlling white balance adjustment.
A further object of the invention is to provide a color image pickup system which does not cause missed photography opportunities and never causes any faulty signal to be recorded in a system capable of effectively using control data stored in a memory arrangement.
Still another object of the invention is to provide a color image pickup system wherein the validity of control data stored in a memory arrangement is judged and determined to minimize the necessity of troublesome white balance adjustment.
It is another object of the invention to provide a color image pickup system which makes a display according to control data stored in a memory arrangement to inform the operator of the degree of existing white balance prior to an image pickup operation.
An additional object of the invention is to provide a color image pickup system which minimizes power consumption.
These and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.